Study On The Pore Structure And Seepage Characteristics Of The Shanxi Formation In The Daning-Jixian Area Of Eastern Ordos

The Ordos Basin’s Da Ning-Jixian block boasts abundant natural gas resources and promising exploration prospects.However,there is limited understanding of the tight sandstone reservoir’s micro-characteristics in the study area,such as pore-throat structure,fluid occurrence,and flow behavior.This study employed observation of core samples,physical property tests,casting thin sections,scanning electron microscopy,grain size analysis,X-ray diffraction,and other testing methods to investigate the petrological characteristics and diagenesis of the reservoir.High-pressure mercury injection and nuclear magnetic resonance experiments were used to reveal micro-pore structure characteristics,pore types,sizes,distribution,and sorting of the Shanxi Formation reservoir.The influence of grain size and diagenesis on pore structure was analyzed,and fractal dimension was introduced to characterize pore structure heterogeneity and explore its relationship with pore structure parameters.The study quantitatively researched the microscopic permeability characteristics through nuclear magnetic resonance and displacement experiments and investigated the factors that affect permeability characteristics.The main findings are:1.The Shanxi Formation reservoir mainly comprises of quartz sandstone,with calcite and dolomite as the primary cements.The reservoir has abundant diagenetic types,including compaction,cementation,and dissolution,with compaction and cementation being the primary factors that reduce porosity.2.The tight sandstone reservoir’s pore types in the Shanxi Formation are predominantly laminar and sheet-like throats,with point-like throats and constricted throats being relatively uncommon.The presence of clay minerals leads to the development of tubular throats.Based on high-pressure mercury injection parameters and capillary pressure curve shape,the reservoir’s pore structure is classified into five types,ranging from the best（Type I）to the worst（Type V）.The pore radius decreases from Type I to Type V,and the sorting becomes poorer.3.The pore structure of the Shanxi Formation reservoir is primarily influenced by grain size,clay minerals,and diagenesis.Larger rock particle sizes correspond to larger pore radii,while smaller rock particle sizes lead to more complex pore distributions and smaller pore-throat radii.Clay minerals generally have a negative effect on the pore structure of the rock.Compaction and cementation have negative impacts on the reservoir’s pore structure in the study area,while dissolution has a partially constructive effect on the pore structure.4.The mercury injection capillary pressure curve and nuclear magnetic resonance fractal curve of the tight sandstone reservoir in the Shanxi Formation show a complex distribution,and the fractal dimension is affected by mineral content.The larger the fractal dimension,the stronger the heterogeneity of the reservoir.Large pore throats have a significant impact on reservoir heterogeneity,and the fractal dimension of large pore throats has a good correlation with FZI,RQI,R_T35,and R_T50,which are good parameters for characterizing pore structure heterogeneity.5.Bound water in the Shanxi Formation is mainly distributed in pores and throats smaller than100 nm.Permeability decreases as bound water saturation increases,and the contribution of small pores and throats to bound water increases as permeability decreases.Bound water saturation is positively correlated with pore size and negatively correlated with clay mineral content and pore structure complexity.6.Gas-water phase seepage experiments show that tight sandstone reservoirs in the Shanxi Formation can be classified into four types:Irp to IVrp.The relative permeability curves gradually worsen from type Irp to type IVrp,corresponding to different pore structure types.As pore structure worsens,the sample’s permeability decreases,and pore size and sorting are the main factors affecting fluid flow capacity.